The present invention relates to equipment for processing semiconductor wafers, and in particular to a method and apparatus for processing two or more wafers at the same time in two or more processing chambers.
Numerous techniques and apparatus are well known for use in the processing of semiconductor wafers to make integrated circuits. The state of the art fabrication facilities (known as “fabs”) for carrying out such processes are typically large buildings within which “clean rooms” of thousands of square feet of floor area are provided. The clean rooms contain the equipment within which the various semiconductor fabrication processes are carried out, for example, chemical vapor deposition equipment for deposition of conductive or insulative materials on the wafers, ion implantation equipment for introduction of impurities into the wafers, furnaces for heating the wafers, plasma etchers for removing material from the wafers, etc.
Compared even to their recent predecessors, clean rooms today are extraordinarily clean, often having particle densities of less than class 1. Such low particle densities require expensive equipment to purify the air in the clean room, as well as unusual care in all other respects. The result of these measures is that floor space in such clean rooms is expensive. The per-square-foot construction cost, as well as maintenance cost, is high.
Another trend in the manufacture of integrated circuits is the use of single wafer processing equipment. In single wafer equipment, processing is carried out on the wafers one wafer at a time. That is, one wafer is introduced from a cassette holding many wafers into the processing chamber. The necessary process on the wafer is carried out in the chamber, then the wafer is removed from the chamber and the next wafer introduced. Typically, such single wafer processing chambers are clustered around a central robot which can load the chambers with individual wafers. The use of single wafer processing provides higher yields by making the process more controllable across the entire wafer, typically 8 inches in diameter, with 12 inches in the near future. The higher yields produced by single wafer systems have resulted in their use in many of the advanced fabrication facilities used today in the semiconductor industry.
In some cases, it is desirable to perform the same process in a plurality of chambers that are stacked together in a multideck arrangement or clustered together in a side-to-side arrangement. These chambers may share the same resources such as, for example, a process gas system, a vacuum pumping system, a radio frequency system, and a control system. To achieve identical wafer processing results in the chambers can be difficult. For instance, the control of process gas flow through the chambers is important to achieve matching of film properties formed on the wafers in the respective chambers. The gases from the process gas system may not split equally among the plurality of chambers. The volume of gas flowing into each chamber is dependent on the flow conductance through that chamber. Flow conductance is in turn dependent on manufacturing tolerances of the chamber components including the input manifold, mixing insert, blocker plate, faceplate, and the like. Perfect matching of the chamber components is difficult. The use of mass flow controllers for the chambers to compensate for manufacturing tolerances is unsuitable if the precursor for the process gas is a liquid (e.g., TEOS) due to condensation problems.